A process for forming electroformable microstructures having a triangular or trapezoidal cross-section from a resist material is disclosed in the publication of J. Mohr, W. Ehrfeld and D. Munchmeyer, Kernforschungszentrum Karlsruhe, Report KfK-4414, entitled "Analysis of the Causes of Defects and the Accuracy of Structure Reproduction in Deep-Etch X-Ray Lithography using Synchroton Radiation" called the "LIGA process". Report KfK-4414 is herein incorporated by reference.
In the above-noted publication, the manufacture of microstructures having slanted sidewalls, i.e., sidewalls slanted relative to the plane of the resist layer, is described. In Chapter 2, the effects on the quality of the fabricated microstructures is described, when employing a mask which is positioned at an angle relative to the incident radiation and which has mask openings which allow radiation to strike the resist layer.
Thus, microstructures that have slanted sidewalls relative to the resist plane, and in principle, microstructures having a triangular or trapezoidal cross-section, can be manufactured by positioning (e.g. by tilting) the plane of the mask at an angle relative to the incident radiation. According to KfK-4414, however, this method results in the attenuation of the radiation dose at the edge region of the mask openings. As a result, portions of the resist material which correspond to the edge region of the microstructures to be fabricated are only partially irradiated by means of reflection and diffraction effects.
As a consequence, when developing the resist structures, the partially irradiated portions of the resist material become partially detached. During subsequent electroplating, the microstructures thus obtained are deformed and have inaccurate dimensions.
Furthermore, KfK-4414 also describes an investigation relating to the influence of masks that are arranged orthogonally to the incident radiation, with mask openings having walls slanted relative to the incident radiation.
According to page 86 of KfK-4414, an attractive possibility for the manufacture of conical microstructures having a small angle of conicity is described. However, this proposal is disadvantageous because the mask is to be fabricated with mask openings having slanted walls, such that the conicity of the resulting microstructures are restricted to small angles. Furthermore, a new mask having designedly slanted mask opening walls must be fabricated for each desired microstructure.